The present invention relates to a method of reproducing gradated images such as half-tone images or the like.
There are generally known a dither method and a density pattern method as a method of reproducing the gradations of image information. In these methods, a reference matrix pattern is formed by (n.times.m) unit pixels, a predetermined ones of the (n.times.m) unit pixels, which are included in the reference matrix pattern, are turned on (for example turned to "black") according to density of the given image information, and the density is represented in accordance with the ratio of the unit pixels turned on in the reference matrix pattern (for example black regions) and the ratio of the unit pixels turned off (for example white regions) and thereby the gradations are reproduced.
By way of example, if a 3.times.3 reference matrix pattern is used, the (3.times.3+1=10) gradations can be reproduced. If a 4.times.4 reference matrix pattern is used, (4.times.4+1=17) gradations can be reproduced.
When the gradations are reproduced using the dither method or the density pattern method, it is important the gradation reproducing capability and resolution are contrary to each other. Namely, if it is desirable to increase the number of gradations, the number of unit pixels in the reference matrix pattern should be increased. However, the reference matrix pattern is thereby made larger and thus the resolution of images is deteriorated.
To eliminate the defect above-mentioned, i.e. to increase the number of gradations and not to deteriorate the resolution, a method is executed wherein each pixel, which configures the reference matrix pattern, is further divided into fine pixels to make a group thereof and the fine pixels are gradually grown, that is, the number of fine pixels turned on are gradually increased and thereby the gradations are reproduced.
By way of example, in a laser beam printer, the unit pixel can be further divided into a plurality of fin pixels by controlling lighting time of a laser beam width modulation or controlling the quantity of light of the laser by light intensity modulation. In a LED printer, the fine pixels can be obtained by changing lighting pulse width or light intensity of a LED. Furthermore, in a liquid crystal printer, the fine pixels can be obtained by changing the pulse width of transmitted light or the quantity of the transmitted light.
Conventionally, there are two methods in case of reproducing the gradations with use of the fine pixels.
(1) In a first method, the thresholds are assigned so that the fine pixels may be grown in ranks shown in FIG. 11. Namely, each fine pixel in the unit pixels 101 with the reference matrix pattern is grown (the ranks, in which the unit pixels are turned on, are expressed by 1, 2, 3, 4 . . . ). When the growth of all the fine pixels in the unit pixels 101 is terminated, the fine pixels in the unit pixels 102 adjacent to the right side of the unit pixels 101 are serially grown (See JP, A-214662/1986).
According to the threshold ranking of fine pixels above-mentioned, there is a following advantage.
In the case that the gradations are reproduced by using the fine pixels, a method of printing in an actual printing output may be unstable and thereby the gradations may not be faithfully reproduced. The size of the fine pixels may be easily unstable and the resolution of images is deteriorated, for example, due to bleeding and the like in the printer with the use of an ink-jet method or thermal transferring method, while due to spreading of toner and the like in the printer with the use of an electrophotographic method.
According to the threshold ranking of fine pixels above-mentioned, however, the fine pixels, which are arranged on a plurality of oblique lines L at regular intervals, are gradually grown with a plurality of reference matrix patterns arranged. The fine pixels are not first adapted to be grown in a position out of the oblique lines L. Accordingly, even if the method of printing becomes unstable, there is little possibility that the unit pixels, which are adjacent to each other, will be overlapped due to the bleeding, the spreading or the like.
There is a following defect in the threshold ranking, wherein all the fine pixels in the unit pixels 101 are first grown, thereby the entire unit pixels 101 are turned on and thereafter the fine pixels in the unit pixels 102 adjacent to the unit pixels 101 are serially grown as described above.
Since the fine pixels in the unit pixels are first grown, there is a good possiblity that the unit pixels, of which all the fine pixels are turned off (that is, white unit pixels), will appear. If there are a lot of unit pixels of which all the fine pixels are turned off, line drawing information on characters, fine lines or the like cannot be reproduced well. The unit pixels, of which all the fine pixels are turned off, become, for example, white dots. If there are a lot of white dots, the line drawings are interrupted therein. Therefore, it is difficult to read the characters, while it is impossible to precisely reproduced fine lines or the like.
(2) There has been proposed a second method of outputting images in JP, A-214665/1986, wherein the gradations can be more faithfully reproduced with use of the fine pixels. According to the method of outputting images as above-mentioned, the gradations are reproduced by distributedly growing the fine pixels in the reference matrix pattern at a low density region, while collectively growing a plurality of fine pixels at a high density region.
There will be more particularly discussed the method of outputting images with reference to FIG. 12A. In FIG. 12A, the reference matrix pattern is configured by (3.times.3=9) unit pixels. Each unit pixel is divided into five fine pixels.
In the case that the gradations are reproduced, each fine pixel is sequentially turned on from the lowest number (threshold) assigned thereto. If the gradations are heightened by one at the high density region, three fine pixels are simultaneously turned on.
According to the method of outputting images as above-mentioned, however, the number of gradations is theoretically decreased. The reason is that when the gradations are reproduced at the high density region, a plurality of fine pixels are collectively increased in accordance with the change of gradations.
In FIG. 12B, a pattern having 45 gradations is reproduced on the basis of the reference matrix pattern, to which the threshold ranks in FIG. 12A, are assigned. As seen from FIG. 12B, even if the number of gradations is theoretically 46, only 34 gradations can be reproduced in practice.
If the numbers of growing fine pixels are different at the high density region unlike the middle and low density regions each time the gradations are heightened by one, the reproduction of gradations is caused to be dependent upon an output device considerably. Therefore, the available output device is limited.
In the prior art above-mentioned, the threshold ranks may be partially assigned to each fine pixel in the unit pixels configuring the reference matrix pattern or the number of gradations may be different. Therefore, in the case that image data, which has a sudden change of gradations of line drawing information or the like, is reproduced, the reproducing efficiency cannot be obtained. Further, there is possibility that the reproduced images will be largely changed due to a method of applying the image information to the reference matrix pattern.
More specifically, if the positional relation between original picture data and the matrix pattern is such as to be shown in FIG. 13A, that is, the gradations of image data, which are given to each unit pixels in the matrix pattern, are expressed by numerical values as shown, the image pattern is reproduced as shown in FIG. 13B.
On the other hand, if the positional relation between the image data and the matrix pattern is such as to be shown in FIG. 13C, that is, the positions are shifted upwardly as compared to FIG. 13A, the image pattern is reproduced as shown in FIG. 13D.
As apparent from the comparison of the image pattern of FIG. 13B with that of FIG. 13D, if the method of outputting images above-mentioned is used, the image pattern to be reproduced is largely changed according to the positional relation between the image data and the pattern.
There has been also considered a method of distributedly increasing the number of fine pixels one by one according to the change of gradations even in the case that the gradations are reproduced at the high density region. According to the method above-mentioned, however, there are a lot of unit pixels, in which only one fine pixel turned off at the high density region remains, in the reference matrix pattern. Accordingly, the matrix pattern may be easily affected by the bleeding or spreading of toner or the like and thereby the gradations cannot be stably reproduced.
It is an object of the present invention to provide a method of reproducing gradations wherein a plurality of unit pixels configuring the reference matrix pattern are respectively formed with a plurality of fine pixels, and the gradations are reproduced in accordance with the ratio of the fine pixels turned on, so that the gradations can be faithfully reproduced according to the density of images and the higher resolution can be obtained.
It is another object of the present invention to provide a method of reproducing gradations wherein threshold ranking of the fine pixels is improved.
It is a further object of the present invention to provide a method of reproducing gradations wherein the number of gradiations is not theoretically decreased and the matrix pattern is seldom affected by the bleeding or spreading of toner or the like.